The present invention relates to a process and device for the treatment of a molten metal or alloy by passing it through a complex bed formed from granules of solid flux immersed in a bath of liquid flux. It is used particularly in the founding of light metals and alloys and, in particular, of aluminium and it alloys, magnesium and its alloys and of some other metals whose fairly low melting points are compatible with the use of conventional metal halide based fluxes such as lead, tin or zinc.
Hereinafter, the word "treatment" will designate all the operations to which the raw molten metal or alloy is subjected before it is used in the foundry in order to eliminate the impurities and, in particular, the inclusions which would impair the properties of the cast articles. These operations can be of a purely physical nature such as filtration or a physico-chemical nature such as the placing in contact with fluxes.
The need to filter the metal so as to obtain sound articles which are free from defects has been recognized both in the founding of articles and in the semicontinuous casting of plates, billets or ingots and in continuous casting. In practice, mechanical filtration has been combined with "washing" using a flux which usually constitutes one or more halides of molten alkali and alkaline earth metals which makes it easier to "wet" the inclusions and to collect them in the form of slag.
The inclusions which are not wetted by the metal and are wetted by the flux arriving at an interface thus created between the molten and the liquid flux are trapped at this interface and penetrate the liquid flux. They are removed from the molten metal during this "washing."
The more developed the interface between the molten metal and liquid flux (fine dispersion or emulsion) and the more the liquids are mixed, the more effective is the treatment carried out for a given quantity of molten metal to be treated and a given quantity of flux, owing to the renewal of the liquids in the vicinity of the interface.
Numerous patents describe processes and devices allowing a metal to be both washed by a flux and filtered through porous masses.
In particular, it is possible to cite U.S. Pat. Nos.: 2,863,333, 3,006,473, 3,010,712, 3,025,155, 3,039,864, 3,172,757, 3,821,238, 3,737,305 in the name of Alcoa, British Pat. No. 1,266,500 and French Pat. No. 2,061,246 in the name of Baco (British Aluminium Company), French Pat. Nos. 1,254,899 and 1,258,674 and British Pat. No. 1,148,344 of Foseco, U.S. Pat. No. 3,227,547 of Union Carbide.
All these patents propose different arrangements for making it easier to bring the metal and liquid flux into contact so as to prevent the slag from being entrained toward the casting ladle and to make the metal pass through solid filtering compositions. We have not mentioned as prior art those patents in which a gas which is sometimes chlorinated is injected into the molten metal.
The various processes enumerated above either involve carrying out mere physical filtration or producing a dispersion of flux in the molten metal and stirring it by mechanical means or by bubbling an inert or active gas through it. These processes have been considered satisfactory for numerous years. At present, however, the requirements of aeronautical design, of thin strips for cans, of thin sheets, of fine wires are forcing the producers of semi-finished products made of metals and light alloys to improve the quality infinitely, and the refining of methods of analysis has shown that the microscropic inclusions, which had been considered negligible up until now, could impair certain mechanical characteristics and increase the amount of waste and, sometimes, render certain advanced thermal treatments, carried out in the vicinity of the melting point of the eutectic constituents of the alloys, inoperative or even harmful.